Coding device and method, decoding device and method, recording medium, and program

ABSTRACT

An encoding apparatus and an encoding method, a decoding apparatus and a decoding method, a recording medium, and a program suitable for encoding image signals with a higher compression ratio for transmission or accumulation. In an arithmetic coding section, from among the syntax elements of input image compression information, the frame/field flag is first encoded by a frame/field flag context model. When the macroblock to be processed is subjected to frame-based encoding, a frame-based context model, specified in the current H.26L standard, is applied. On the other hand, when the macroblock to be processed is subjected to field-based encoding, a field-based context model is applied for the syntax elements described below. The present invention is applied to an encoder for encoding image information and a decoder for decoding image information.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application of U.S. application Ser.No. 12/468,917, filed May 20, 2009, which is a continuation applicationof U.S. application Ser. No. 10/509,682, filed Oct. 12, 2004, which isnow U.S. Pat. No. 7,778,331, which is a National Stage application ofPCT/JP03/05081 filed on April 22, 2003, and claims priority to JapanesePatent Application No. JP2002-125295, filed on Apr. 26, 2002. The entirecontents of the above-identified applications are incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to an encoding apparatus and an encodingmethod, a decoding apparatus and a decoding method, a recording medium,and a program. The present invention relates to, for example, anencoding apparatus and an encoding method, a decoding apparatus and adecoding method, a recording medium, and a program suitable for encodingimage signals with a higher compression ratio for transmission oraccumulation.

BACKGROUND ART

Nowadays, apparatuses in compliance with, for example, MPEG (MovingPicture Expert Group), which is an image compression standard based onorthogonal transformation, such as discrete cosine transformation, andmotion compensation where redundancies specific to image information areexploited to handle images as digital signals for efficient transmissionand accumulation of such digital signals, are being widely used for bothinformation distribution by broadcast stations and information receptionin households.

In particular, the MPEG2 (ISO/IEC 13818-2) compression technique is astandard defined as a general-purpose image compression scheme, coveringinterlaced scan images and progressive scan images, as well asstandard-resolution images and high-definition images. Thus, MPEG2 iswidely used by both professionals and general consumers, as seen in, forexample, the DVD (Digital Versatile Disk) standards.

The use of the MPEG2 compression scheme accomplishes a high compressionratio and high image quality by assigning bit rates of, for example, 4to 8 Mbps for interlaced scan images with a standard resolution of720×480 pixels and bit rates of, for example, 18 to 22 Mbps forinterlaced scan images with a high resolution of 1920×1088 pixels.

Since MPEG2 is mainly intended for a high quality encoding techniquesuitable for broadcasting, it does not support an encoding scheme for ahigher compression ratio. This is the reason the MPEG4 encoding systemhas been standardized as an encoding scheme for a higher compressionratio. The image encoding scheme was approved as an internationalstandard ISO/IEC 14496-2 in December 1998.

Furthermore, the standardization of H.26L (ITU-T Q6/16 VCEG), originallyintended for image encoding for video conferences, is being promoted byITU-T (International Telecommunication Union-TelecommunicationStandardization Sector).

H.26L is known as a standard which achieves a higher encodingefficiency, though it requires a larger amount of arithmetic operationfor encoding processing and decoding processing compared with knownencoding schemes such as MPEG2 and MPEG4.

In addition, one of the current MPEG4 activities includes Joint Model ofEnhanced-Compression Video Coding, being promoted jointly with ITU-T,for the standardization of an encoding scheme which achieves a higherencoding efficiency based on H.26L and employs functions not supportedby H.26L.

A known image information encoding apparatus based on orthogonaltransformation, such as discrete cosine transformation or Karhunen-Loevetransform, and motion compensation will now be described with referenceto FIG. 1. FIG. 1 shows an example structure of a known imageinformation encoding apparatus.

In the relevant image information encoding apparatus, an input imagesignal, as an analog signal, is converted to a digital signal by an A/Dconversion section 1 and the digital signal is then passed to a picturesorting buffer 2. The picture sorting buffer 2 rearranges frames of theimage information from the A/D conversion section 1 according to the GOP(Group of Pictures) structure of the image compression informationoutput by the relevant image information encoding apparatus.

Images that are subjected to intra-encoding (encoding in an image) willfirst be described. In the picture sorting buffer 2, the imageinformation of an image to be subjected to intra-encoding is passed toan orthogonal transformation section 4 via an adder 3.

In the orthogonal transformation section 4, the image information issubjected to orthogonal transformation (e.g., discrete cosinetransformation or Karhunen-Loeve transform), and the obtained transformcoefficient is passed to a quantization section 5. In the quantizationsection 5, the transform coefficient supplied from the orthogonaltransformation section 4 is subjected to quantization processing underthe control of a rate control section 8 based on the amount of transformcoefficient data accumulated in an accumulation buffer 7.

In a lossless encoding section 6, an encoding mode is determined basedon the quantized transform coefficient, quantization scale, etc.supplied from the quantization section 5, and the determined encodingmode is subjected to lossless encoding (e.g., variable-length encodingor arithmetic coding) to form information to be stored in the header ofan image encoding unit. Furthermore, the encoded encoding mode issupplied to the accumulation buffer 7 for accumulation. The encodedencoding mode accumulated in the accumulation buffer 7 is output to thesubsequent stage as image compression information.

In addition, in the lossless encoding section 6, the quantized transformcoefficient is subjected to lossless encoding and the encoded transformcoefficient is accumulated in the accumulation buffer 7. The encodedtransform coefficient, accumulated in the accumulation buffer 7, is alsooutput to the subsequent stage as image compression information.

In a dequantization section 9, the transform coefficient quantized bythe quantization section 5 is dequantized. In an inverse orthogonaltransformation section 10, the dequantized transform coefficient issubjected to inverse orthogonal transformation processing and decodedimage information is generated. The generated decoded image informationis accumulated in a frame memory 11.

Images that are subjected to inter-encoding (encoding between images)will now be described. In the picture sorting buffer 2, the imageinformation of an image to be subjected to inter-encoding is supplied tothe adder 3 and a motion prediction/compensation section 12.

In the motion prediction/compensation section 12, image information forreference that corresponds to the image from the picture sorting buffer2 that is subjected to inter-encoding is read out from the frame memory11 and then subjected to motion prediction/compensation processing togenerate reference image information, which is then supplied to theadder 3. Furthermore, motion vector information obtained as a result ofmotion prediction/compensation processing in the motionprediction/compensation section 12 is supplied to the lossless encodingsection 6.

In the adder 3, the reference image information from the motionprediction/compensation section 12 is converted to a differential signalfrom the image information of the image from the picture sorting bufferthat is subjected to inter-encoding.

When an image which is subjected to inter-encoding is to be processed,the differential signal is subjected to orthogonal transformation in theorthogonal transformation section 4, and the obtained transformcoefficient is supplied to the quantization section 5. In thequantization section 5, the transform coefficient supplied from theorthogonal transformation section 4 is subjected to quantizationprocessing under the control of the rate control section 8.

In the lossless encoding section 6, an encoding mode is determined basedon the transform coefficient and the quantization scale quantized by thequantization section 5, as well as the motion vector informationsupplied from the motion prediction/compensation section 12 and otherinformation. The determined encoding mode is then subjected to losslessencoding to generate information to be stored in the header of an imageencoding unit. The encoded encoding mode is accumulated in theaccumulation buffer 7. The encoded encoding mode accumulated in theaccumulation buffer 7 is output as image compression information.

Furthermore, in the lossless encoding section 6, motion vectorinformation from the motion prediction/compensation section 12 issubjected to lossless encoding processing to generate information to bestored in the header of the image encoding unit.

When an image which is subjected to inter-encoding is to be processed,the processing in the dequantization section 9 and the subsequentprocessing are carried out in the same manner as with intra-encoding,and will not be described.

A known image information decoding apparatus which receives imagecompression information output by the known image information encodingapparatus shown in FIG. 1 to restore an image signal will now bedescribed with reference to FIG. 2. FIG. 2 shows an example structure ofa known image information decoding apparatus.

In the relevant image information decoding apparatus, image compressioninformation which has been input is temporarily stored in anaccumulation buffer 21 and transferred to a lossless decoding section22. The lossless decoding section 22 applies lossless decoding (e.g.,variable-length decoding or arithmetic decoding) to the imagecompression information based on a predetermined format of imagecompression information to acquire the encoding mode information storedin the header and supplies it to a dequantization section 23. Thelossless decoding section 22 also acquires the quantized transformcoefficient to supply it to the dequantization section 23. Furthermore,if the frame to be decoded has been subjected to inter-encoding, thelossless decoding section 22 also decodes the motion vector informationstored in the header of the image compression information and suppliesthe information to a motion prediction/compensation section 28.

The dequantization section 23 dequantizes the quantized transformcoefficient supplied from the lossless decoding section 22, and suppliesthe obtained transform coefficient to an inverse orthogonaltransformation section 24. The inverse orthogonal transformation section24 applies inverse orthogonal transformation (e.g., inverse discretecosine transformation or inverse Karhunen-Loeve transform) to thetransform coefficient based on a predetermined format of the imagecompression information.

If the relevant frame has been subjected to intra-encoding, the imageinformation subjected to inverse orthogonal transformation is stored ina picture sorting buffer 26 via an adder 25, converted to an analogsignal by a D/A conversion section 27, and then output to the subsequentstage. The image information subjected to inverse orthogonaltransformation is also stored in a frame memory 29.

Furthermore, if the relevant frame has been subjected to inter-encoding,a reference image is generated in the motion prediction/compensationsection 28 based on the motion vector information from the losslessdecoding section 22 and the image information stored in the frame memory29 and is then supplied to the adder 25. In the adder 25, the referenceimage from the motion prediction/compensation section 28 is combinedwith the output from the inverse orthogonal transformation section 25 togenerate image information. The other processing is carried out in thesame manner as with a frame subjected to intra-encoding and will not bedescribed.

According to H.26L, two types of encoding: UVLC (Universal VariableLength Code), one type of variable-length encoding, and CABAC(Context-based adaptive binary arithmetic coding), one type ofarithmetic coding, are defined as lossless encoding schemes. Thus, theuser can select one of UVLC and CABAC as a lossless encoding scheme. Theinformation indicating whether the lossless encoding scheme used is UVLCor CABAC is specified in the field called Entropy Coding included in theRTP Parameter Set Packet of the RTP layer in the image compressioninformation.

Arithmetic coding, to which CABAC belongs, will now be described. Inarithmetic coding, any message (including a plurality of alphabeticsymbols) is represented as one point in a semi-open interval 0.0≦x<1.0,and the code is generated based on the coordinates of this point.

First, the semi-open interval 0.0≦x<1.0 is divided into subintervals,each corresponding to a symbol, on the basis of the occurrenceprobabilities of the symbols included in the alphabetic sequence.

FIG. 3 shows an example of the occurrence probabilities of symbols s₁ tos₇ with their respective subintervals. In arithmetic coding, the upperlimit and the lower limit of a subinterval are determined on the basisof the cumulative occurrence probability of each symbol, as shown inFIG. 3. The lower limit of the subinterval for the symbol s_(i) (i=1, 2,. . . , 7) is equal to the upper limit of the subinterval for thepreceding symbol s_(i-1), and the upper limit of the subinterval for thesymbol s_(i) is equal to the value obtained by adding the occurrenceprobability of the symbol s_(i) to the lower limit of the subintervalfor the symbol s_(i).

Let us assume that (s₂s₁s₃s₆s₇) is input as a message. Here, the symbols₇ is assumed to be a terminal symbol which represents the end of themessage. In short, the message ends with this terminal symbol. Thearithmetic coding scheme calculates a subinterval corresponding to eachsymbol included in the message (s₂s₁s₃s₆s₇), as shown in FIG. 4. Inother words, the interval assigned as shown in FIG. 3 is divided inproportion to the cumulative occurrence probability of the subsequentsymbol. The subinterval obtained finally is the range which includes thevalue representing the message. In this manner, any value in this rangecan uniquely restore the corresponding message. It is noted, however,that a value that can be represented by a power of two in the semi-openinterval is used to represent the message, taking the encodingefficiency into consideration.

More specifically, in this example, the value obtained by Expression (2)shown below represents the message included in the semi-open interval0.211645≦x<0.2117 on the basis of Expressions (1) shown below.

$\begin{matrix}{{2^{- 1} = 0.5}{2^{- 2} = 0.25}{2^{- 3} = 0.125}{2^{- 4} = 0.0625}{2^{- 5} = 0.03125}{2^{- 6} = 0.015625}{2^{- 7} = 0.0078125}{2^{- 8} = 0.00390625}{2^{- 9} = 0.001953125}{2^{- 10} = 0.0009765625}{2^{- 11} = 0.00048828125}{2^{- 12} = 0.000244140625}\vdots} & (1) \\{{2^{- 3} + 2^{- 4} + 2^{- 6} + 2^{- 7} + 2^{- 11} + 2^{- 12}} = 0.211669921875} & (2)\end{matrix}$

Thus, a code length of 12 bits is sufficient for the length of the codecorresponding to the message (s₂s₁s₃s₆s₇) so that a value from 2⁻¹ to2⁻¹² can be represented to encode the message (s₂s₁s₃s₆s₇) into(001101100011).

CABAC defined in H.26L will now be described. Details of CABAC aredescribed in a document “Video Compression Using Context-Based AdaptiveArithmetic Coding”, Marpe et al, ICIO1 (hereinafter, referred to asDocument 1). CABAC has the following three features, compared with UVLC,which is also defined in H.26L.

A first feature is a capability of eliminating the redundancy betweensymbols by using a context model appropriate for each symbol to beencoded to carry out arithmetic coding based on an independentprobability model.

A second feature is a capability of assigning a bit rate of anon-integer value to each symbol in arithmetic coding, i.e., acapability of achieving an encoding efficiency similar to that ofentropy.

For example, statistical data of a motion vector is variable in spaceand time, as well as with respect to bit rates and sequences. A thirdfeature enables encoding in response to such variations to be carriedout by applying adaptive encoding.

FIG. 5 shows a typical structure of a CABAC encoder to which CABAC isapplied. In the relevant CABAC encoder, a context modeling section 31first converts the symbol of any syntax element in image compressioninformation to an appropriate context model according to the history.Such modeling is called context modeling. The context model for eachsyntax element in image compression information will be described below.

A binarization section 32 binarizes a symbol which is not binarized. Inan adaptive binary arithmetic coding section 33, the binarized symbol isthen subjected to probability estimation by a probability estimationsection 34, and is subjected to adaptive arithmetic coding by anencoding engine 35 based on the probability estimation. After adaptivearithmetic coding processing has been carried out, the related modelsare updated, and each model can carry out encoding processing accordingto the statistics of actual image compression information.

Here, context models for carrying out arithmetic coding of MB_type(MB_type), motion vector information (MVD), and reference frameparameter (Ref_frame), which are syntax elements in image compressioninformation, will now be described.

Context model generation for MB_type will be described for each of twocases: a case of intra-frame and a case of inter-frame.

If macroblocks A, B, and C are arranged as shown in FIG. 6 on anintra-frame, the context model ctx_mb_type_intra(C) corresponding to theMB_type of the macroblock C is defined according to Expression (3) shownbelow. The mode of a macroblock on an intra-frame is Intra4×4 orIntra16×1.

ctx_mb_type_intra(C)=A+B  (3)

In Expression (3), A is 0 when the macroblock A is Intra4×4 or 1 whenthe macroblock A is Intra16×16. Similarly, B is 0 when the macroblock Bis Intra4×4 or 1 when the macroblock B is Intra16×16. Therefore, thecontext model ctx_mb_type_intra(C) takes one of 0, 1, and 2.

If the macroblocks A, B, and C are arranged as shown in FIG. 6 on aninter-frame which is a P picture, the context model ctx_mb_type_inter(C)corresponding to the MB_type of the macroblock C is defined according toExpression (4) shown below. If the relevant inter-frame is a B picture,the context model ctx_mb_type_inter(C) is defined according toExpression (5) shown below.

ctx_mb_type_inter(C)=((A==Skip)?0:1)+((B==Skip)?0:1)  (4)

ctx_mb_type_inter(C)=((A==Direct)?0:1)+((B==Direct)?0:1)  (5)

In Expression (4), the operator ((A==Skip)?0:1) indicates 0 if themacroblock A is in the Skip mode or 1 if the macroblock A is not in theSkip mode. Similarly, the operator ((B==Skip)?0:1) indicates 0 if themacroblock B is in the Skip mode or 1 if the macroblock B is not in theSkip mode.

In Expression (5), the operator ((A==Direct)?0:1) indicates 0 if themacroblock A is in the Direct mode or 1 if the macroblock A is not inthe Direct mode. Similarly, the operator ((B==Direct)?0:1) indicates 0if the macroblock B is in the Direct mode or 1 if the macroblock B isnot in the Direct mode.

Therefore, there are three types of the context modelctx_mb_type_inter(C) corresponding to the MB_type of the macroblock C onan inter-frame (P picture) for each of the P picture and the B picture.

Context model generation for motion vector information (MVD) will now bedescribed.

Motion vector information corresponding to the macroblock of interestincluded in image compression information is encoded as predictionerrors from the motion vector corresponding to the neighboringmacroblocks. The evaluation function e_(k)(C) for the macroblock C ofinterest, from among the macroblocks A, B, and C arranged as shown inFIG. 7, is defined according to Expression (6) shown below. InExpression (6), k=0 indicates the horizontal component, whereas k=1indicates the vertical component.

e _(k)(C)=|mvd_(k)(A)|+|mvd_(k)(B)|  (6)

Here, mvd_(k)(A) and mvd_(k)(B) indicate motion vector prediction errorswith respect to the macroblocks A and B, respectively, neighboring themacroblock C.

In Expression (6), if the macroblock C is disposed at the left edge ofthe picture frame, i.e., if one of the macroblocks A and B does notexist, information related to the corresponding motion vector predictionerror mvd_(k)(A) or mvd_(k)(B) cannot be obtained, and hence thecorresponding item in the right-hand member of Expression (6) isignored. The context model ctx_mvd(C,k) corresponding to e_(k)(C)defined as described above is defined according to Expressions (7-1) to(7-3) below.

ctx_mvd(C,k)=0 e _(k)(C)<3  (7-1)

ctx_mvd(C,k)=1 32<e _(k)(C)  (7-2)

ctx_mvd(C,k)=2 3≦e _(k)(C)≦32  (7-3)

Context model generation for the motion vector information (MVD) iscarried out as shown in FIG. 8. More specifically, the motion vectorprediction error mvd_(k)(C) for the macroblock C is divided into theabsolute value |mvd_(k)(C)| and the sign. The absolute value|mvd_(k)(C)| is binarized. The first bin (the leftmost value) of thebinarized absolute value |mvd_(k)(C)| is encoded using theabove-described context model ctx_mvd(C,k). The second bin (the secondvalue from the left) is encoded using context model 3. Similarly, thethird and fourth bins are encoded using context models 4 and 5,respectively. The fifth bin and the subsequent bins are encoded usingcontext model 6. The sign of mvd_(k)(C) is encoded using context model7. As described above, motion vector information (MVD) is encoded usingeight types of context models.

Context models for encoding the reference frame parameter (Ref_frame)will now be described.

When two or more reference frames are used for an inter-frame,information related to the reference frame is set for each macroblock ofthe inter-frame. If the reference frame parameters for the macroblocks Aand B are represented as A and B, respectively, with respect to themacroblocks A, B, and C arranged as shown in FIG. 6, the context modelctx_ref_frame(C) for the macroblock C is defined according to Expression(8) shown below.

ctx_ref_frame(C)=((A==0)?0:1)+2((B==0)?0:1)  (8)

In Expression (8), the operator ((A==0)?0:1) is 0 when the referenceframe parameter for the macroblock A is 0 or 1 when the reference frameparameter for the macroblock A is not 0. Similarly, the operator((B==0)?0:1) is 0 when the reference frame parameter for the macroblockB is 0 or 1 when the reference frame parameter for the macroblock B isnot 0.

Thus, four types of context models for encoding the reference frameparameter (Ref_frame) are defined according to Expression (8).Furthermore, the context model for the second bin and the context modelsfor the third bin and the subsequent bins are defined.

Context models for arithmetically encoding the code block pattern (CBP),which is a syntax element related to the texture information included inthe image compression information according to H.26L, theintra-prediction mode (IPRED), and the (RUN,LEVEL) information will nowbe described.

The description starts with context models related to the code blockpattern. The handling of code block patterns other than an Intra16×16macroblock is defined as follows.

That is, as the CBP bits for the luminance signal, one CBP bit isincluded in each of four 8×8 blocks of an Intra16×16 macroblock, i.e., atotal of four CBP bits. When the macroblocks A, B, and C are arranged asshown in FIG. 6, the context model ctx_cbp_luma(C) corresponding to theluminance signal of the macroblock C is defined according to Expression(9) shown below.

ctx_cbp_luma(C)=A+2B  (9)

In Expression (9), A indicates the CBP bit of the luminance signal ofthe macroblock A, and B indicates the CBP bit of the luminance signal ofthe macroblock B.

The remaining two bits in the CBP field are related to the chrominancesignal. The context model ctx_cbp_chroma_sig(C) corresponding to thechrominance signal of the macroblock C is defined according toExpression (10) shown below.

ctx_cbp_chroma_sig(C)=A+2B  (10)

In Expression (10), A indicates the CBP bit of the chrominance signal ofthe macroblock A, and B indicates the CBP bit of the chrominance signalof the macroblock B.

Here, if the context model ctx_cbp_chroma_sig(C) corresponding to thechrominance signal of the macroblock C is not 0, i.e., if the ACcomponents of the chrominance signal exist, the context modelctx_cbp_chroma_ac(C) corresponding to the AC components of thechrominance signal of the macroblock C defined according to Expression(11) shown below needs to be encoded.

ctx_cbp_chroma_ac(C)=A+2B  (11)

In Expression (11), A indicates the cbp_chroma_ac decision correspondingto the macroblock A, and B indicates the cbp_chroma_ac decisioncorresponding to the macroblock B.

Since the context models defined according to Expressions (9) to (11)are defined separately for the intra-macroblock and theinter-macroblock, a total of 24 (=2×3×4) types of context models aredefined.

Furthermore, in the case of an Intra16×16 macroblock, one type ofcontext model is defined for the binarized AC decision, and one type ofcontext model is defined for each component of the chrominance signal.

Context models related to the intra-prediction mode (IPRED) will now bedescribed. Six types of intra-prediction modes (label 0 to 5) defined inH.26L will now be described with reference to FIGS. 9 and 10. FIG. 9shows pixels a to p existing in a 4×4 block generated by dividing amacroblock and pixels A to I existing in the neighboring 4×4 blocks.Labels 1 to 5 in FIG. 10 indicate intra-prediction modes with differentdirections. The intra-prediction mode indicated by label 0 is a DCprediction mode (DC Prediction).

In the intra-prediction mode of label 0, the pixels a to p are predictedaccording to Expression (12) shown below.

pixels a to p=(A+B+C+D+E+F+G+H)//8  (12)

In Expressions (12) to (15), A to I indicate the pixels A to I,respectively, and the symbol “//” means an arithmetic operation suchthat the result of division is rounded off.

In the intra-prediction mode indicated by label 0, if four pixels (e.g.,the pixels A to D) of the eight pixels A to H do not exist in thepicture frame, Expression (12) is not used and the mean value of theremaining four pixels (the pixels E to H n this case) is used aspredicted values for the pixels a to p. Furthermore, if none of theeight pixels A to H exists in the picture frame, Expression (12) is notused and a predetermined value (e.g., 128) is used as predicted valuesof the pixels a to p.

The intra-prediction mode indicated by label 1 is calledVertical/Diagonal Prediction. The intra-prediction mode of label 1 isused only when the four pixels A to D exist in the picture frame. Inthis case, the pixels a to p are predicted according to Expressions(13-1) to (13-6) shown below.

pixel a=(A+B)//2  (13-1)

pixel e=B  (13-2)

pixels b,i=(B+C)//2   (13-3)

pixels f,m=C  (13-4)

pixels c,j=(C+D)//2  (13-5)

pixels d,g,h,k,l,n,o,p=D  (13-6)

The intra-prediction mode indicated by label 2 is called VerticalPrediction. The intra-prediction mode of label 2 is used only when thefour pixels A to D exist in the picture frame. In this case, the pixel Ais used as predicted values of, for example, the pixels a, e, i, and m,and the pixel B is used as predicted values of, for example, the pixelsb, f, j, and n.

The intra-prediction mode indicated by label 3 is called DiagonalPrediction. The intra-prediction mode of label 1 is used only when thenine pixels A to I exist in the picture frame. In this case, the pixelsa to p are predicted according to Expressions (14-1) to (13-7) shownbelow.

pixel m=(H+2G+F)//4   (14-1)

pixels i,n=(G+2F+E)//4  (14-2)

pixels e,j,o=(F+2E+I)//4  (14-3)

pixels a,f,k,p=(E+2I+A)//4  (14-4)

pixels b,g,l=(I+2A+B)//4  (14-5)

pixels c,h=(A+2B+C)//4  (14-6)

pixel d=(B+2C+D)//4  (14-7)

The intra-prediction mode indicated by label 4 is called HorizontalPrediction. The intra-prediction mode of label 4 is used only when thefour pixels E to H exist in the picture frame. In this case, the pixel Eis used as predicted values of, for example, the pixels a, b, c, and d,and the pixel F is used as predicted values of, for example, the pixelse, f, g, and h.

The intra-prediction mode indicated by label 5 is calledHorizontal/Diagonal Prediction. The intra-prediction mode of label 5 isused only when the four pixels E to H exist in the picture frame. Inthis case, the pixels a to p are predicted according to Expressions(15-1) to (15-6) shown below.

pixel a=(E+F)//2  (15-1)

pixel b=F  (15-2)

pixels c,e=(F+G)//2  (15-3)

pixels f,d=G  (15-4)

pixels i,g=(G+H)//2  (15-5)

pixels h,j,k,l,m,n,o,p=H  (15-6)

Two context models are defined for each of the intra-prediction modes oflabels 0 to 5. More specifically, one of the two context models is thefirst bin for each mode and the other of the two context models is thesecond bin for each mode. In addition to these context models, onecontext model is defined for each of the two bits in the Intra16×16mode. Therefore, a total of 14 context models are defined for theintra-prediction mode.

Context models related to (RUN,LEVEL) will now be described.

In H.26L, two types of scan methods shown in FIGS. 11A and 11B aredefined as methods for rearranging a two-dimensional discrete cosinetransform coefficient into a one-dimensional coefficient. The singlescan technique shown in FIG. 11A is used for the luminance signal of anintra-macroblock in a case other than that where the quantizationparameter QP is smaller than 24. The double scan technique shown in FIG.11B is used when the single scan technique is not used.

In an inter-macroblock and an intra-macroblock with a quantizationparameter QP of 24 or larger, an average of one non-zero coefficientexists for a 4×4 macroblock, in short, a one-bit EOB (End Of Block)signal is sufficient. For the luminance signal of an intra-macroblockwith a quantization parameter QP smaller than 24, two or more non-zerocoefficients exist, and a one-bit EOB signal is not sufficient. This isthe reason that the double scan technique shown in FIG. 11B is used.

As shown in FIG. 12, nine types of context models are defined for(RUN,LEVEL) according to the discrimination of the above-described scanmethod, the discrimination between DC block type and AC block type, thediscrimination between luminance signal and chrominance signal, and thediscrimination between intra-macroblock and inter-macroblock.

The LEVEL information is separated into the sign and the absolute value.Four context models are defined according to the correspondingCtx_run_level shown in FIG. 12. More specifically, the first contextmodel is defined for the sign, the second context model is defined forthe first bin, the second context model is defined for the second bin,and the fourth context model is defined for the subsequent bins.

When LEVEL is not 0 (i.e., the LEVEL is not an EOB), RUN described belowis encoded. For RUN, two context models are defined for eachCtx_run_level shown in FIG. 12: one for the first bin and the other forthe second and subsequent bins.

Context models for the quantization-related parameter Dquant that can beset at the macroblock level in image compression information accordingto H.26L will now be described.

The parameter Dquant is set when the code block pattern for themacroblock includes a non-zero orthogonal transform coefficient or themacroblock is 16×16 Intra Coded. The parameter Dquant can range from −16to 16. The quantization parameter QUANT_(new) for the macroblock iscalculated according to Expression (16) shown below that uses theparameter Dquant in the image compression information.

QUANT_(new)=modulo₃₂(QUANT_(old)+Dquant+32)  (16)

In Expression (16), QUANT_(old) is the quantization parameter used forthe previous encoding or decoding.

The first context model ctx_dquant(C) for the parameter Dquant of themacroblock C arranged as shown in FIG. 6 is defined according toExpression (17) shown below.

ctx_dquant(C)=(A!=0)  (17)

In Expression (17), A indicates the value of the parameter Dquant of themacroblock A. The second context model is defined for the first bin andthe second context model is defined for the second and the subsequentbins.

If a symbol which is input to the context models described above is notbinarized, the symbol must be binarized before it can be input to thecontext models. Syntax elements other than MB_type are binarizedaccording to the relationships shown in FIG. 13.

MB_type, ten types of which are defined for the P picture, is binarizedaccording to the relationship shown in FIG. 14A. Furthermore, MB_type,17 types of which are defined for the B picture, is binarized accordingto the relationships shown in FIG. 14B.

Registers for the above-described various context models arepre-initialized with pre-calculated values, and when a symbol is to beencoded, the occurrence frequencies of the bins for a series of contextmodels are successively updated for a determination in the encoding ofthe subsequent symbol.

If the occurrence frequency for a given context model exceeds apredetermined value, the frequency counter is scaled down. Through suchperiodic scaling processing, dynamic occurrence of symbols can behandled easily.

For the arithmetic coding scheme for binarized symbols in H.26L, theapproach disclosed in a document “Arithmetic Coding for DataCompression”, (Witten et al. Comm. of the ACM, 30 (6), 1987, pp520-541)(hereinafter, referred to as Document 2) is applied, as of this writing.

In MPEG2, if an image signal to be input is of interlaced scan format,field/frame adaptive encoding processing can be carried out at themacroblock level.

Although such specifications are not defined in H.26L at present, adocument “Interlace Coding Tools for H.26L Video Coding (L. Wang et al.,VCEG-O37, December 2001)” (hereinafter, referred to as Document 3)proposes that the H.26L specifications be extended to supportfield/frame adaptive encoding processing at the macroblock level.

The field/frame adaptive encoding processing at the macroblock levelproposed in Document 3 will now be described.

According to the current H.26L, seven types of modes (modes 1 to 7), asshown in FIG. 15, are defined as units of motion prediction/compensationin a macroblock.

Document 3 proposes that a frame/field flag be disposed between Run andMB_type as the syntax corresponding to the macroblock in imagecompression information, as shown in FIG. 16. If the value of theframe/field flag is 0, it indicates that the relevant macroblock is tobe subjected to frame-based encoding. In contrast, if the value of theframe/field flag is 1, it indicates that the relevant macroblock is tobe subjected to field-based encoding.

If the value of the frame/field flag is 1, i.e., if field-based encodingis to be applied, the pixels in the macroblock are rearranged row byrow, as shown in FIG. 17.

If the value of the frame/field flag is 1, five types of modes (modes 1ato 5a), as shown in FIG. 18, i.e., the five types of modes correspondingto the modes 3 to 7 in FIG. 15, are defined as units of motionprediction/compensation in the macroblock.

For example, in the mode 2a of FIG. 18, the blocks 0 and 1 out of thefour 8×8 blocks 0 to 3 generated by dividing the macroblock belong tothe same field parity, and the blocks 2 and 3 belong to the same fieldparity. Furthermore, for example, in the mode 3a of FIG. 18, the blocks0 to 3 of the eight 4×8 blocks 0 to 8 generated by dividing themacroblock belong to the same field parity, and the blocks 4 to 7 belongto the same field parity.

The intra-prediction mode when the value of the frame/field flag is 1will now he described. For example, the pixels a to p disposed in the4×4 block shown in FIG. 9 are subjected to intra-prediction using thepixels A to I disposed in the neighboring 4×4 blocks, also when thevalue of the frame/field flag is 1. In this case, it should be notedthat all of the pixels a to p and the pixels A to I belong to the samefield parity.

A description when the pixels A to I and the pixels a to p belong to thesame macroblock will now be given with reference to FIG. 19. The pixelsa to p existing in the 4×4 block 7 generated by dividing the macroblockinto 16 are subjected to intra-prediction using the pixels A to Idisposed at the edges of the neighboring blocks 2, 3, and 6.

A description when the pixels A to I belong to a macroblock differentfrom that of the pixels a to p will now be given with reference to FIGS.20A and 20B.

FIG. 20A shows that the frame/field flag values of the macroblocks tothe left of and above the macroblock for processing are 1. In this case,the intra-prediction of the pixels existing in the 4×4 block C generatedby dividing the target macroblock into 16 is carried out based on thepixels in the 4×4 block A generated by dividing the macroblock to theleft into 16 and the pixels in the 4×4 block B generated by dividing themacroblock above into 16. The intra-prediction of the pixels existing inthe 4×4 block C′ is carried out based on the pixels existing in the 4×4block A′ and the pixels existing in the 4×4 block B′.

FIG. 20B shows an example where the value of the frame/field flag forthe target macroblock for processing is 1 and the values of theframe/field flags for the macroblocks to the left and above are 0. Inthis case, the intra-prediction of the pixels existing in the 4×4 blockC generated by dividing the target macroblock into 16 is carried outbased on the pixels in the 4×4 block A generated by dividing themacroblock to the left into 16 and the pixels in the 4×4 block Bgenerated by dividing the macroblock above into 16. The intra-predictionof the pixels existing in the 4×4 block C′ is carried out based on thepixels existing in the 4×4 block A′ and the pixels existing in the 4×4block B′.

Intra-prediction of the chrominance signal will now be described withreference to FIG. 21. When the value of the frame/field flag is 1, onlyone type of intra-prediction mode for the chrominance signal is defined.

A to D in FIG. 21 each represent a 4×4 block of the chrominance signal.The blocks A and B belong to the first field and the blocks C and Dbelong to the second field. s₀ to s₂ are the sum of the chrominancesignals existing in the blocks which belong to the first field parityand neighbor the blocks A to D. s₃ to s₅ are the sum of the chrominancesignals existing in the blocks which belong to the second field parityand neighbor the blocks A to D.

The predicted values A to D respectively corresponding to the blocks Ato D are predicted according to Expressions (18) shown below providedthat s₀ to s₅ all exist in the picture frame.

A=(s ₀ +s ₂+4)/8

B=(s ₁+2)/4

C=(s ₃ +s ₅+4)/8

D=(s ₄+2)/4   (18)

If only s₀, s₁, s₃, and s₄ of s₀ to s₅ exist in the picture frame, thepredicted values A to D respectively corresponding to the blocks A to Dare predicted according to Expressions (19) shown below.

A=(s ₀+2)/4

B=(s ₁+2)/4

C=(s ₃+2)/4

D=(s ₄+2)/4  (19)

Furthermore, if only s₂ and s₅ of s₀ to s₅ exist in the picture frame,the predicted values corresponding to the blocks A to D are predictedaccording to Expressions (20) shown below.

A=(s ₂+2)/4

B=(s ₂+2)/4

C=(s ₅+2)/4

D=(s ₅+2)/4  (20)

FIG. 22 shows a method for encoding the residual components of thechrominance signal after intra-prediction has been applied as describedabove. More specifically, each of the 4×4 blocks is subjected toorthogonal transformation processing, the 2×2 blocks as shown in thefigure are generated using the DC components of the first field and thesecond field, and orthogonal transformation processing is again applied.

Motion prediction/compensation processing when the value of theframe/field flag is 1 will now be described. When the value of theframe/field flag is 1, there are six types of motionprediction/compensation modes: an inter-16×16 mode, an inter-8×16 mode,an inter-8×8 mode, an inter-4×8 mode, and an inter-4×4 mode.

For example, the inter-16×16 mode is a mode in which the motion vectorinformation for the first field, the motion vector information for thesecond field, and the reference frame in the inter-8×16 mode areequivalent.

These six types of motion prediction/compensation modes are respectivelyassigned Code_Numbers 0 to 5.

In the current H.26L, a multiple-frame prediction for allowing aplurality of reference frames as shown in FIG. 23 to be provided isspecified. In the current frame-based H.26L standard, informationrelated to reference frames is defined at the macroblock level such thatthe previously encoded frame is assigned Code_Number 0, and the framesone to five times preceding the frame with Code_Number 0 arerespectively assigned Code_Number 1 to Code_Number 5.

On the other hand, for field-based encoding, the first field of thepreviously encoded frame is assigned Code_Number 0, and the second fieldof the same frame is assigned Code_Number 1. The first field of theframe preceding the frame with Code_Number 0 is assigned Code_Number 2and the second field of the relevant frame is assigned Code_Number 3.The first field of the frame preceding the frame with Code_Number 2 isassigned Code_Number 4 and the second field of the relevant frame isassigned Code_Number 5.

Furthermore, for macroblocks that are subjected to field-based encoding,the reference field for the first field and the reference field for thesecond field are specified separately from each other.

The median prediction specified in the current H.26L will now bedescribed with reference to FIG. 24, followed by the description of amotion vector information prediction method when the value of theframe/field flag is 1. The 16×16, 8×8, or 4×4 motion vector informationcorresponding to the 16×16 macroblock E shown in FIG. 24 is predictedusing the median of the motion vector information of the neighboringmacroblocks A to C.

Any of the macroblocks A to C that does not exist in the picture frame,however, is assumed to have a motion vector information value of 0 formedian calculation. If, for example, the macroblocks D, B, and C do notexist in the picture frame, the motion vector information correspondingto the macroblock A is used as the predicted value. Furthermore, if themacroblock C does not exist in the picture frame, the median iscalculated using the motion vector information of the macroblock Dinstead of the macroblock C.

The reference frames for the macroblocks A to D do not need to be thesame.

A description when the block size of the macroblock is 8×16, 16×8, 8×4,or 4×8 will now be given with reference to FIGS. 25A to 25D. Themacroblock E of interest and the neighboring macroblocks A to D areassumed to be arranged as shown in FIG. 24.

FIG. 25A shows an example where the block sizes of the macroblocks E1and E2 are 8×16. For the left-hand macroblock E1, if the neighboringmacroblock A to the left refers to the same frame as the macroblock E1,the motion vector information of the macroblock A is used as thepredicted value. If the neighboring macroblock A to the left refers to aframe different from that referred to by the macroblock E1, theabove-described median prediction is applied.

For the right-hand macroblock E2, if the neighboring macroblock C to theupper right refers to the same frame as the macroblock E2, the motionvector information of the macroblock C is used as the predicted value.If the neighboring macroblock C to the upper right refers to a framedifferent from that referred to by the macroblock E2, theabove-described median prediction is applied.

FIG. 25B shows an example where the block sizes of the macroblocks E1and E2 are 16×8. For the upper macroblock E1, if the neighboringmacroblock B above refers to the same frame as the macroblock E1, themotion vector information of the macroblock B is used as the predictedvalue. If the neighboring macroblock B above refers to a frame differentfrom that referred to by the macroblock E1, the above-described medianprediction is applied.

For the lower macroblock E2, if the neighboring macroblock A to the leftrefers to the same frame as the macroblock E2, the motion vectorinformation of the macroblock A is used as the predicted value. If theneighboring macroblock A to the left refers to a frame different fromthat referred to by the macroblock E2, the above-described medianprediction is applied.

FIG. 25C shows an example where the block sizes of the macroblocks E1 toE8 are 8×4. The above-described median prediction is applied for theleft-hand macroblocks E1 to E4, and the motion vector information of theleft-hand macroblocks E1 to E4 is used as the predicted values for theright-hand macroblocks E5 to E8.

FIG. 25D shows an example where the block sizes of the macroblocks E1 toE8 are 4×8. The above-described median prediction is applied for theupper macroblocks E1 to E4, and the motion vector information of theupper macroblocks E1 to E4 is used as the predicted values for the lowermacroblocks E5 to E8.

Also, if the value of the frame/field flag is 1, the horizontaldirection component of the motion vector information is predicted incompliance with the above-described method. For the vertical directioncomponent, however, a field-based block and a frame-based block aremixed, and the following processing is carried out. The macroblock E ofinterest and the neighboring macroblocks A to D are assumed to bearranged as shown in FIG. 24.

When the macroblock E is to be subjected to frame-based encodingprovided that one of the neighboring macroblocks A to D has beensubjected to field-based encoding, the mean value between the verticaldirection component of the motion vector information for the first fieldand the vertical direction component of the motion vector informationfor the second field is multiplied by two, and the result is used as anequivalent to the frame-based motion vector information for predictionprocessing.

When the macroblock E is to be subjected to field-based encodingprovided that one of the neighboring macroblocks A to D has beensubjected to frame-based encoding, the vertical direction componentvalue of the motion vector information is divided by two, and the resultis used as an equivalent to the field-based motion vector informationfor prediction processing.

According to Document 3, a syntax element necessary for field/frameencoding at the macroblock level is added, and furthermore, thesemantics of a syntax element such as motion vector information ischanged. Nevertheless, in Document 3, no new context model is introducedor an existing context model is not updated in response to theabove-described addition and change. Thus, the information provided inDocument 3 is not sufficient to carry out field/frame encoding at themacroblock level using the CABAC scheme.

CABAC is known as a scheme which achieves a higher encoding efficiency,though it requires a larger amount of arithmetic operation for encodingprocessing compared with UVLC, and therefore it is preferable that CABACis available for field/frame encoding at the macroblock level even wheninput image information has an interlaced scan format.

DISCLOSURE OF INVENTION

In view of the situation described above, an object of the presentinvention is to enable field/frame encoding at the macroblock level tobe performed using the CABAC scheme even when input image informationhas an interlaced scan format.

An encoding apparatus according to the present invention includeslossless encoding means for carrying out lossless encoding processingusing a context model corresponding to a frame/field flag indicatingwhether the encoding processing at the macroblock level is field-basedor frame-based, a context model corresponding to a syntax element forcarrying out the frame-based encoding processing, and a context modelcorresponding to a syntax element for carrying out the field-basedencoding processing.

The context model corresponding to the syntax element for carrying outthe field-based encoding processing may include at least one of thecontext models corresponding to an MB_type for an I picture, an MB_typefor a P/B picture, motion vector information, a reference fieldparameter, and an intra-prediction mode.

An encoding method according to the present invention includes alossless encoding step of carrying out lossless encoding processingusing a context model corresponding to a frame/field flag indicatingwhether the encoding processing at the macroblock level is field-basedor frame-based, a context model corresponding to a syntax element forcarrying out the frame-based encoding processing, and a context modelcorresponding to a syntax element for carrying out the field-basedencoding processing.

A program on a first recording medium according to the present inventionincludes a lossless encoding step of carrying out lossless encodingprocessing using a context model corresponding to a frame/field flagindicating whether the encoding processing at the macroblock level isfield-based or frame-based, a context model corresponding to a syntaxelement for carrying out the frame-based encoding processing, and acontext model corresponding to a syntax element for carrying out thefield-based encoding processing.

A first program according to the present invention enables a computer toexecute a lossless encoding step of carrying out lossless encodingprocessing using a context model corresponding to a frame/field flagindicating whether the encoding processing at the macroblock level isfield-based or frame-based, a context model corresponding to a syntaxelement for carrying out the frame-based encoding processing, and acontext model corresponding to a syntax element for carrying out thefield-based encoding processing.

A decoding apparatus according to the present invention includesdecoding means for decoding image compression information that isencoded using a context model corresponding to a frame/field flagindicating whether the encoding processing at the macroblock level isfield-based or frame-based, a context model corresponding to a syntaxelement for carrying out the frame-based encoding processing, and acontext model corresponding to a syntax element for carrying out thefield-based encoding processing.

A decoding method according to the present invention includes a decodingstep of decoding image compression information that is encoded using acontext model corresponding to a frame/field flag indicating whether theencoding processing at the macroblock level is field-based orframe-based, a context model corresponding to a syntax element forcarrying out the frame-based encoding processing, and a context modelcorresponding to a syntax element for carrying out the field-basedencoding processing.

A program on a second recording medium according to the presentinvention includes a decoding step of decoding image compressioninformation that is encoded using a context model corresponding to aframe/field flag indicating whether the encoding processing at themacroblock level is field-based or frame-based, a context modelcorresponding to a syntax element for carrying out the frame-basedencoding processing, and a context model corresponding to a syntaxelement for carrying out the field-based encoding processing.

A second program according to the present invention enables a computerto execute a decoding step of decoding image compression informationthat is encoded using a context model corresponding to a frame/fieldflag indicating whether the encoding processing at the macroblock levelis field-based or frame-based, a context model corresponding to a syntaxelement for carrying out the frame-based encoding processing, and acontext model corresponding to a syntax element for carrying out thefield-based encoding processing.

In the encoding apparatus, encoding method, and first program accordingto the present invention, lossless encoding is carried out using acontext model corresponding to a frame/field flag indicating whether theencoding processing at the macroblock level is field-based orframe-based, a context model corresponding to a syntax element forcarrying out the frame-based encoding processing, and a context modelcorresponding to a syntax element for carrying out the field-basedencoding processing.

In the decoding apparatus, decoding method, and second program accordingto the present invention, image compression information that is encodedusing a context model corresponding to a frame/field flag indicatingwhether the encoding processing at the macroblock level is field-basedor frame-based, a context model corresponding to a syntax element forcarrying out the frame-based encoding processing, and a context modelcorresponding to a syntax element for carrying out the field-basedencoding processing is decoded.

The encoding apparatus and the decoding apparatus may be apparatusesindependent of each other or may be a block for carrying out encodingand decoding in a signal processing apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the structure of a known imageinformation encoding apparatus for carrying out image compression byorthogonal transformation and motion compensation.

FIG. 2 is a block diagram showing the structure of an image informationdecoding apparatus corresponding to the image information encodingapparatus in FIG. 1.

FIG. 3 is a diagram showing an example of the relationship between theoccurrence probabilities of symbols and their respective subintervals inarithmetic coding.

FIG. 4 is a diagram showing an example of arithmetic coding.

FIG. 5 is a block diagram showing a typical structure of a CABACencoder.

FIG. 6 is a diagram illustrating a context model for MB_type.

FIG. 7 is a diagram illustrating a context model for motion vectorinformation MVD.

FIG. 8 is a diagram illustrating the encoding of motion vectorinformation MVD based on a context model.

FIG. 9 is a diagram illustrating an intra-prediction mode defined inH.26L.

FIG. 10 is a diagram illustrating the directions of the intra-predictionmodes indicated by labels 1 to 5.

FIG. 11A is a diagram illustrating the single scan technique defined inH.26L.

FIG. 11B is a diagram illustrating the double scan technique defined inH.26L.

FIG. 12 is a diagram showing a context model corresponding to(RUN,LEVEL) defined in H.26L.

FIG. 13 is a diagram illustrating the binarization of syntax elementsother than MB_type in H.26L.

FIG. 14A is a diagram illustrating the binarization of MB_type of the Ppicture in H.26L.

FIG. 14B is a diagram illustrating the binarization of MB_type of the Bpicture in H.26L.

FIG. 15 is a diagram showing seven types of modes defined in H.26L as aunit of motion prediction/compensation in a macroblock.

FIG. 16 is a diagram showing syntax for image compression informationextended such that field/frame adaptive encoding can be carried out atthe macroblock level.

FIG. 17 is a diagram illustrating the rearrangement of pixels of amacroblock when the macroblock is subjected to field-based encoding.

FIG. 18 is a diagram showing five types of modes defined as a unit ofmotion prediction/compensation when a macroblock is subjected tofield-based encoding.

FIG. 19 is a diagram illustrating the operating principle forintra-prediction in a macroblock when the macroblock is subjected tofield-based encoding.

FIG. 20A is a diagram illustrating the operating principle forintra-prediction across macroblocks when a macroblock is subjected tofield-based encoding.

FIG. 20B is a diagram illustrating the operating principle forintra-prediction across macroblocks when a macroblock is subjected tofield-based encoding.

FIG. 21 is a diagram illustrating the operating principle forintra-prediction for the chrominance signal when a macroblock issubjected to field-based encoding.

FIG. 22 is a diagram illustrating the operating principle for encodingthe residual components of the chrominance signal when a macroblock issubjected to field-based encoding.

FIG. 23 is a diagram illustrating a multiple-frame prediction specifiedin H.26L.

FIG. 24 is a diagram illustrating a method for predicting motion vectorinformation when a macroblock is subjected to field-based encoding.

FIG. 25A is a diagram illustrating the generation of predicted values ofmotion vector information in prediction modes specified in H.26L.

FIG. 25B is a diagram illustrating the generation of predicted values ofmotion vector information in prediction modes specified in H.26L.

FIG. 25C is a diagram illustrating the generation of predicted values ofmotion vector information in prediction modes specified in H.26L.

FIG. 25D is a diagram illustrating the generation of predicted values ofmotion vector information in prediction modes specified in H.26L.

FIG. 26 is a block diagram showing an example structure of an imageinformation encoding apparatus according to an embodiment of the presentinvention.

FIG. 27 is a block diagram showing an example structure of thearithmetic coding section 58 in FIG. 26.

FIG. 28A is a diagram showing a table for binarizing the MB_type of amacroblock belonging to a P picture when the macroblock is subjected tofield-based encoding.

FIG. 28B is a diagram showing a table for binarizing the MB_type of amacroblock belonging to a B picture when the macroblock is subjected tofield-based encoding.

FIG. 29 is a block diagram showing an example structure of an imageinformation decoding apparatus according to an embodiment of the presentinvention, the decoding apparatus corresponding to the image informationencoding apparatus in FIG. 26.

BEST MODE FOR CARRYING OUT THE INVENTION

An image information encoding apparatus to which the present inventionis applied will now be described with reference to FIG. 26. The relevantimage information encoding apparatus enables encoding to be performedusing the CABAC scheme even when input image information has aninterlaced scan format.

In the relevant image information encoding apparatus, an A/D conversionsection 51 converts an input image signal as an analog signal to adigital signal and outputs it to a picture sorting buffer 52. Thepicture sorting buffer 52 rearranges the input image information fromthe A/D conversion section 51 according to the GOP structure of theimage compression information which is output from the relevant imageinformation encoding apparatus and outputs it to an adder 54.

A field/frame determination section 53 determines which of frame-basedencoding and field-based encoding provides a higher encoding efficiencyto encode the macroblock of the image to be processed, generates theappropriate frame/field flag, and outputs the result to a field/frameconversion section 55 and an arithmetic coding section 58.

When the macroblock to be processed is subjected to inter-encoding, theadder 54 generates a differential image between the input image via thefield/frame determination section 53 and the reference image from amotion prediction/compensation section 64, and outputs the differentialimage to the field/frame conversion section 55 and to an orthogonaltransformation section 56. On the other hand, when the macroblock to beprocessed is subjected to intra-encoding, the adder 54 outputs the inputimage via the field/frame determination section 53 as-is to thefield/frame conversion section 55 and to the orthogonal transformationsection 56.

When the macroblock to be processed is subjected to field-basedencoding, the field/frame conversion section 55 converts the input imagefrom the adder 54 into a field structure, and outputs the result to theorthogonal transformation section 56. The orthogonal transformationsection 56 applies orthogonal transformation (e.g., discrete cosinetransformation or Karhunen-Loeve transform) to the input imageinformation, and supplies the obtained transform coefficient to aquantization section 57. The quantization section 57 appliesquantization processing to the transform coefficient supplied from theorthogonal transformation section 56 under the control of the ratecontrol section 65.

The arithmetic coding section 58 arithmetically encodes each syntaxelement input from the quantization section 57 and the motionprediction/compensation section 64, as well as the frame/field flag fromthe field/frame determination section 53, based on the CABAC scheme, andsupplies the results to an accumulation buffer 59 for accumulation. Theaccumulation buffer 59 outputs the accumulated image compressioninformation to the subsequent stage.

A dequantization section 60 dequantizes the quantized orthogonaltransform coefficient and outputs it to an inverse orthogonaltransformation section 61. The inverse orthogonal transformation section61 applies inverse orthogonal transformation processing to thedequantized transform coefficient, generates decoded image information,and supplies it to a frame memory 62 for accumulation. When themacroblock to be processed is subjected to field-based encoding, afield/frame conversion section 63 converts the decoded image informationaccumulated in the frame memory 62 into a field structure, and outputsit to the motion prediction/compensation section 64.

The motion prediction/compensation section 64 generates the optimalprediction mode information and the motion vector information throughmotion prediction processing and outputs it to the arithmetic codingsection 58. Furthermore, the motion prediction/compensation section 64generates a predicted image to output it to the adder 54. A rate controlsection 65 performs feedback control of the operation of thequantization section 57 based on the amount of data accumulated in theaccumulation buffer 59. A control section 66 controls each section ofthe relevant image information encoding apparatus according to a controlprogram recorded on a recording medium 67.

The operating principle of the arithmetic coding section 58 will now bedescribed with reference to FIG. 27. FIG. 27 shows an example structureof the arithmetic coding section 58. From among the syntax elements ofthe input image compression information, the frame/field flag shown inFIG. 16 is first encoded by a frame/field flag context model 91 in thearithmetic coding section 58.

When the macroblock to be processed is subjected to frame-basedencoding, a frame-based context model 92, specified in the current H.26Lstandard, is applied. For syntax elements having a non-binarized value,such a value is binarized by a binarization section 93 and arithmeticcoding is then applied.

On the other hand, when the macroblock to be processed is subjected tofield encoding, a field-based context model 94 is applied for the syntaxelements described below. For syntax elements having a non-binarizedvalue, such a value is binarized by a binarization section 95 andarithmetic coding is then applied. More specifically, the first syntaxelement is MB_type for the I picture, the second syntax element isMB_type for the P/B picture, the third syntax element is motion vectorinformation, the fourth syntax element is a reference field parameter,and the fifth syntax is an intra-prediction mode.

The following description assumes that the macroblocks A, B, and C arearranged as shown in FIG. 6. Context models related to the frame/fieldflag will now be described. The context model ctx_fifr_flag(C) relatedto the frame/field flag of the macroblock C is defined according toExpression (21) shown below.

ctx_fifr_flag(C)=a+2b  (21)

In Expression (21), a and b are the values of the frame/field flags ofthe macroblocks A and B, respectively.

Context models related to MB_type for the I picture will now bedescribed. When the frame/field flag is 1, the context modelctx_mb_type_intra_field(C) corresponding to the MB_type of themacroblock C included in the I picture is defined according toExpression (22) shown below, as with Expression (3).

ctx_mb_type_intra_field(C)=A+B  (22)

A and B in Expression (22) are the same as the respective counterpartsin Expression (3). It does not matter whether the neighboringmacroblocks A and B are subjected to field-based encoding or frame-basedencoding.

Context models related to the MB_type for the P/B picture will now bedescribed. When the macroblock C is included in the P picture, thecontext model ctx_mb_type_inter_field(C) corresponding to the MB_type ofthe macroblock C is defined according to Expression (23) shown below.Furthermore, when the macroblock C is included in the B picture, thecontext model ctx_mb_type_inter_field(C) corresponding to the MB_type ofthe macroblock C is defined according to Expression (24) shown below.

ctx_mb_type_inter_field(C)=((A==skip)?0:1)+2((B==skip)?0:1)  (23)

ctx_mb_type_inter_field(C)=((A==Direct)?0:1)+2((B==Direct)?0:1)  (24)

The operators ((A==skip)?0:1) and ((A==skip)?0:1) in Expression (23) arethe same as those in Expression (4) and the operators ((A==Direct)?0:1)and ((B==Direct)?0:1) in Expression (24) are the same as those inExpression (5). It does not matter whether the neighboring macroblocks Aand B are subjected to field-based encoding or frame-based encoding.

The MB_type of a non-binarized P picture is binarized according to thetable shown in FIG. 28A. Furthermore, the MB_type of a non-binarized Bpicture is binarized according to the table shown in FIG. 28B.

In an adaptive binary arithmetic coding section 96, the binarized symbolis subjected to probability estimation by a probability estimationsection 97, and is subjected to adaptive arithmetic coding based onprobability estimation by the encoding engine 98. The related models areupdated after the adaptive arithmetic coding processing. This enableseach model to carry out encoding processing according to the statisticsof actual image compression information.

For a macroblock that is subjected to frame-based encoding, ten types ofMB_type are defined if the macroblock belongs to the P picture. On theother hand, for a macroblock that is subjected to field-based encoding,the 16×16 mode and the 8×16 mode of the above-described 16 types ofmodels are not defined if the macroblock belongs to the P picture. Inshort, eight types of MB_type are defined for a P-picture-relatedmacroblock that is subjected to field-based encoding.

Eighteen types of MB_type are defined for a B-picture-related macroblockthat is subjected to frame-based encoding. On the other hand, for amacroblock that is subjected to field-based encoding and belongs to theB picture, the forward 16×16 mode, backward 16×16 mode, forward 8×16mode, and backward 8×16 mode from among the above-described 18 types ofmodes are not defined. In short, for a B-picture-related macroblock thatis subjected to field-based encoding, 14 types of MB_type are defined.

Context models for motion vector information will now be described. Whenthe value of the frame/field flag is 1, the first to third contextmodels ctx_mvd_field(C,k) corresponding to the motion vector informationof the macroblock C are defined according to Expressions (25-1) to(25-3) shown below.

ctx_mvd_field(C,k)=0 e _(k)(C)<3  (25-1)

ctx_mvd_field(C,k)=1 32<e _(k)(C)  (25-2)

ctx_mvd_field(C,k)=2 3≦e _(k)(C)≦32  (25-3)

In Expressions (25-1) to (25-3), the evaluation function e_(k) isdefined according to Expression (26) shown below. The macroblocks A andB exist in the same parity field.

e _(k)(C)=|mvd_(k)(A)|+|mvd_(k)(B)|  (26)

If the macroblock A has been subjected to frame-based encoding, mvd₁_(—) _(field)(A) calculated from Expression (27) shown below is appliedto Expression (26) for the motion vector information mvd₁(A) for thevertical direction component. This is also applicable when themacroblock B has been subjected to frame-based encoding.

mvd₁ _(—) _(field)(A)=mvd₁ _(—) _(frame)(A)/2  (27)

In contrast, if the macroblock C is subjected to frame-based encodingand the neighboring block A has been subjected to field-based encoding,mvd_(k) _(—) _(frame)(A) calculated from Expressions (28-1) and (28-2)is applied to Expression (26) respectively for the horizontal directioncomponent and the vertical direction component of mvd_(k)(A).

mvd₀ _(—) _(frame)(A)=(mvd₀ _(—) _(top)(A)+mvd₀ _(—)_(bottom)(A))/2  (28-1)

mvd₁ _(—) _(frame)(A)=mvd₁ _(—) _(top)(A)+mvd₁ _(—) _(bottom)(A)  (28-2)

Context models related to the reference field parameter will now bedescribed. When the value of the frame/field flag is 1, the firstcontext model ctx_ref_field_top(C) corresponding to the first field isdefined according to Expression (29-1) shown below. Furthermore, thefirst context model ctx_ref_field_bot(C) corresponding to the secondfield is defined according to Expression (29-2) shown below.

ctx_ref_field_top(C)=a _(t)+2b _(t)  (29-1)

ctx_ref_field_bot(C)=a _(b)+2b _(b)  (29-2)

In Expressions (29-1) to (29-2), the parameter a_(t) is related to thefirst field of the neighboring macroblock A, the parameter a_(b) isrelated to the second field of the neighboring macroblock A, theparameter b_(t) is related to the first field of the neighboringmacroblock B, and the parameter b_(b) is related to the second field ofthe neighboring macroblock B, as defined in Expressions (30-1) and(30-2) shown below.

a_(t),a_(b),b_(t),b_(b)=0  (30-1)

(when the reference field is the immediate previous encoded field)

a_(t),a_(b),b_(t),b_(b)=1 (otherwise)  (30-2)

Context models corresponding to the second bin and the subsequent binsare each defined in the same manner as with the context modelctx_ref_frame(C) shown in Expression (8). It is noted, however, that theCode_number to be encoded is not assigned to a frame but to a field.

Context models related to an intra-prediction mode will now bedescribed. When the value of the frame/field flag is 1, the contextmodel ctx_intra_pred_field(C) related to the intra-prediction modecorresponding to the macroblock C is defined in the same manner as withthe context model ctx_intra_pred(C) for the macroblock in the framemode. It does not matter whether the neighboring macroblocks A and B aresubjected to field-based encoding or to frame-based encoding.

As described above, field/frame encoding using the CABAC scheme is madepossible by introducing new context models and changing existing contextmodels.

FIG. 29 shows an example structure of an image information decodingapparatus corresponding to the image information encoding apparatus inFIG. 26.

In the relevant image information decoding apparatus, an accumulationbuffer 101 accumulates input image compression information and outputsit to an arithmetic decoding section 102, as required. The arithmeticdecoding section 102 applies arithmetic decoding processing to the imagecompression information encoded based on the CABAC scheme, outputs thedecoded frame/field flag to field/frame conversion sections 105 and 110,outputs the quantized orthogonal transform coefficient to adequantization section 103, and outputs the prediction mode informationand the motion vector information to a motion prediction/compensationsection 111.

The dequantization section 103 dequantizes the quantized orthogonaltransform coefficient decoded by the arithmetic decoding section 102. Aninverse orthogonal transformation section 104 applies inverse orthogonaltransformation to the dequantized orthogonal transform coefficient. Ifthe macroblock to be processed has been subjected to field-basedencoding, the field/frame conversion section 105 converts the outputimage or differential image obtained as a result of inverse orthogonaltransformation into a frame structure.

If the macroblock to be processed is an inter-macroblock, an adder 106combines the differential image from the inverse orthogonaltransformation section 104 and the reference image from the motionprediction/compensation section 111 to generate an output image. Apicture sorting buffer 107 rearranges the output images according to theGOP structure of the input image compression information and outputs itto a D/A conversion section 108. The D/A conversion section 108 convertsthe output image as a digital signal into an analog signal and outputsit to the subsequent stage.

A frame memory 109 stores the image information generated by the adder106, i.e., the image information from which a reference image isgenerated. When the macroblock to be processed has been subjected tofield-based encoding, the field/frame conversion section 110 convertsthe image information stored in the frame memory 111 into a fieldstructure. The motion prediction/compensation section 111 generates areference image from the image information stored in the frame memorybased on the prediction mode information and the motion vectorinformation for each macroblock included in the image compressioninformation, and outputs the reference image to the adder 106.

According to the image information decoding apparatus constructed asdescribed above, image compression information output by the imageinformation encoding apparatus in FIG. 26 can be decoded into theoriginal image information.

The sequence of processing described above can be implemented using notonly hardware but also software. If the sequence of processing is to beimplemented using software, a program constituting the software isinstalled from, for example, a recording medium 67 in FIG. 26 to acomputer built into dedicated hardware or to, for example, ageneral-purpose personal computer that requires programs to be installedto carry out the corresponding functions.

The recording medium 67 may be a package medium including a magneticdisk (including a flexible disk); an optical disk (including a compactdisc-read only memory, i.e., CD-ROM and a digital versatile disk, i.e.,DVD); a magneto-optical disk (including a mini-disc, i.e., MD); or asemiconductor memory if such a program is supplied separately from auser's computer. The recording medium may be a ROM or a hard disk of auser's computer if the program on the recording medium is suppliedpreinstalled on the user's computer.

In the present invention, the steps of programs recorded on therecording medium may or may not be followed time-sequentially in orderof the described steps. Furthermore, the steps may be followed inparallel or independently from one another.

INDUSTRIAL APPLICABILITY

As described above, according to the present invention, field/frameencoding using the CABAC scheme can be carried out even when input imageinformation has an interlaced scan format.

Furthermore, according to the present invention, it is possible torestore image information in an interlaced scan format by decodingcompression image information having image information of interlacedscan format subjected to field/frame encoding using the CABAC scheme atthe macroblock level.

1. A decoding method for decoding received input image compressioninformation, the method comprising: generating a context modelcorresponding to motion vector information of a current macroblock,wherein the context model is generated based on a sum of absolute valuesof first motion vector information of a first macroblock neighboring thecurrent macroblock and absolute values of second motion vectorinformation of a second macroblock neighboring the current macroblock,and when the current macroblock is subjected to the field-basedencoding, the first neighboring macroblock is subjected to frame-basedencoding, and the second neighboring macroblock is subjected toframe-based encoding, calculating the second motion vector informationcorresponding to the second neighboring macroblock by converting avertical component of the second motion vector information correspondingto the second neighboring macroblock to an equivalent for an encodingtype of a first neighboring macroblock, and applying the second motionvector information corresponding to the second neighboring macroblock tothe context model corresponding to the motion vector information of thecurrent macroblock; and decoding the input image compression informationusing the context model corresponding to the syntax element for carryingout the frame-based or the field-based encoding.
 2. A decoder configuredto decode received input image compression information, the decodercomprising: a processor configured to generate a context modelcorresponding to motion vector information of a current macroblock,wherein the context model is generated based on a sum of absolute valuesof first motion vector information of a first macroblock neighboring thecurrent macroblock and absolute values of second motion vectorinformation of a second macroblock neighboring the current macroblock,and when the current macroblock is subjected to the field-basedencoding, the first neighboring macroblock is subjected to frame-basedencoding, and the second neighboring macroblock is subjected toframe-based encoding, the processor is configured to calculate thesecond motion vector information corresponding to the second neighboringmacroblock by converting a vertical component of the second motionvector information corresponding to the second neighboring macroblock toan equivalent for an encoding type of a first neighboring macroblock,and to apply the second motion vector information corresponding to thesecond neighboring macroblock to the context model corresponding to themotion vector information of the current macroblock; and a decodingcircuit configured to decode the input image compression informationusing the context model corresponding to the syntax element for carryingout the frame-based or the field-based encoding.